23Na NMR study of non-superconducting double-layer hydrate NaxCoO2.yH2O

TL;DR

This study uses 23Na NMR to investigate the magnetic and electronic properties of non-superconducting and superconducting double-layer hydrated NaxCoO2.yH2O, revealing hydration effects on Na sites and proximity to quantum criticality.

Contribution

It provides detailed NMR insights into how hydration influences Na sites and spin dynamics, and relates these effects to superconductivity and magnetic instabilities.

Findings

Hydration significantly reduces hyperfine field and electric field gradient at Na sites.

Spin dynamics in non-superconducting hydrate resemble that of non-hydrated Na0.7CoO2.

Superconducting phase is near a quantum critical point with A-type magnetic instability.

Abstract

We report 23Na NMR studies of the polycrystalline samples of double-layer hydrated cobalt oxides NaxCoO2.yH2O (x ~ 0.35 and y ~ 1.3) with the superconducting transition temperatures Tc < 1.8K and ~4.5K, and the dehydrated NaxCoO2 (x ~ 0.35). The hyperfine field and the electric field gradient at the Na sites in the non-hydrated Na0.7CoO2 and the dehydrated Na0.35CoO2 are found to be significantly reduced by the hydration, which indicates a strong shielding effect of the intercalated water molecules on the Na sites. The temperature dependence of 23 Na nuclear spin-lattice relaxation rate 1/23T1 of the non-superconducting double-layer hydrate NaxCoO2.yH2O is found to be similar to that of the non-hydrated Na0.7CoO2, whose spin dynamics is understood by A-type (intra-layer ferromagnetic and inter-layer antiferromagnetic) spin fluctuations. The superconducting phase is located close to the quantum critical point with the A-type magnetic instability.